This invention relates to integrated circuits and more particularly to monolithic integrated circuits of the type having diodes (or other circuit components) formed on a common substrate and which are electrically isolated from each other throughout the substrate.
As used herein, the expression "monolithic integrated circuits" refers to a single substrate or wafer of semiconductive material, typically monocrystalline, on (or in) which individual active and/or passive circuit components are formed which, when appropriately interconnected, results in the desired circuit network. Accordingly, such circuit networks require electrical isolation between the various circuit components formed on or in the wafer.
In the past, various techniques have been developed to provide the electrical components with the required electrical isolation, one from another. For example, in one method, an epitaxial layer of semiconductive material is formed upon a substrate of opposite conductivity type and the discrete components are formed in the epitaxial layer. Thereafter, an isolation region is formed by doping the areas surrounding each device with an impurity having the same conductivity type as the substrate. This technique then requires that the device be heated to an elevated temperature in excess of 1,000.degree. C. for an extended period of time to create vacancies in the lattice structure of the epitaxial layer to enable the impurity to diffuse through the epitaxial layer and contact the substrate, thus forming the isolation barrier.
This prior art technique has a well known disadvantage in that, in diffusing through the epitaxial layer, the dopant tends to diffuse in all directions. The resultant lateral spread of the dopant extends through a larger area of the substrate than would be required if the dopant were made to diffuse in a vertical direction. Additionally, the high temperatures required to enable the dopant to diffuse through the epitaxial layer may, in certain instances, introduce contaminative impurities into the device and thus impair its characteristics.